This invention relates to a light guide unit for use in a liquid crystal display device in which a polarized component of a light is enhanced.
1. Field of the Invention
A polarized light wave is a light in which a component of light is polarized. For obtaining such a polarized light wave, a non-polarized light was conventionally incident to a polarizer plate and either one of the polarized components, i.e., s component and p component, was absorbed. Therefore, more than 50-percent of an incident light was not effectively utilized in principle and an actual measurement shows that about 58-percent of the incident light is absorbed.
Further, a light dispersing sheet having printed dots was typically used in addition to a polarization device for obtaining a polarized light by absorbing a polarized component in a conventional Liquid Crystal Display (LCD) device, and this makes an additional 20-percent of light unavailable.
2. Description of the Related Art
In FIG. 1, a LCD module 100 of a conventional LCD device is shown. The light emanating from a light source 101 transmits through a light guide plate 102 having 96-percent transmittance, a dispersion sheet 103 having 80-percent transmittance, a lower polarizer plate 104 having 42-percent transmittance, a glass substrate 105 having a numerical aperture of 40-percent, a color filter 106 having 30-percent transmittance and an upper polarizer plate 107 having 90-percent transmittance, resulting in an actually available light intensity which is 3.5-percent of the light generated in the light source 101.
This greatly prevents the energy from being utilized efficiently. A back light system of a high intensity for use in a low power consumption LCD device is especially desired because it is an important objective in a portable personal computer to assure a longer usable time for a given capacity of a battery and the power consumption of a back light 108 is a major percentage of a total power consumption.
Also, the light energy absorbed in the lower polarizer plate 104, etc., is converted to a heat energy which contributes to degradation of parts of the LCD device. Particularly for a liquid crystal material of STN (Super Twisted Nematic) type in which the display quality is degraded by heat, it is an important objective to reduce such heat generation. As seen from FIG. 1, 66.4-percent of the light energy is converted to a heat energy by the light absorption in the lower polarizer plate 104 and the dispersion sheet 103 (this is 69-percent of heat generation by the light energy).
A technology of expediting an effective utilization of light is disclosed in PUPA 4-271324 in which a light guide comprises a lamination of a plurality of refracting layers of different refraction indices, and a light incident to an end surface is refracted in each interface between refracting layers so as to reach the light exit surface at an incident angle which is smaller than a critical angle for reducing the decrease of light flux and increasing the efficiency of light utilization.
Japanese patent PUPA 2-201316 uses a liquid crystal cell, a light guide plate disposed in the back of the cell, a colored filter disposed on the light guide plate, a polarizer plate having a reflecting plate provided in the back side of the light guide plate and a light source disposed in the edge surface of the light guide to reduce the number of transmittances through the polarizer plate to 1 thereby improving the efficiency of utilization of the light from the light source.
However, none of these prior art devices directs a light which contains a rich polarized component to the polarizer plate and they are not different from the prior art described above in that a polarized light is obtained by absorbing either one of the s and p components of the light. The art described above does not allow at least a portion of the polarization component which is absorbed and not so far utilized to be utilized. Incidentally, some conventional back lights include about 2.7-percent of light being polarized, but this is not intended to polarize the light from the light source in the back light.
As a technology of obtaining a polarized component by reflection/transmission characteristics without using a polarizer plate, a Polarized Beam Splitter (PBS) and a transmission type linear polarizer exist, but they are only capable of utilizing one of the s and p components of the light or utilizing both separately without a capability of effectively utilizing both s and p components. Further, because a conventional transmission type linear polarizer causes light to be incident directly in Brewster angle, it is unable to emit a condensed light dispersed over a wide exit surface. In addition, a sophisticated work is required for making it into a thin structure. For these reasons, the technology of obtaining a polarized light without using a polarizer plate could not be utilized as a light guide unit for an LCD device.
In order to solve such problems, Japanese patent application 7-40633 was filed by the applicant of the present application relating to a method of improving the efficiency of light utilization in obtaining a polarized light by making at least a portion of a polarized component which was not utilized in the prior art usable for obtaining a polarized light. This application is based on the fact that an s polarized component has reflection/transmission characteristics different from p polarized component so that the reflected light and transmitted light have differing ratios of s and p polarized components, the reflected light having s component of x1-percent and p component of y1-percent. while the transmitted light has an s component of x2-percent and a p component of y2-percent (x1'"x2, y1'"y2). The ratio of s and p components of either the reflected or transmitted light is changed by means of changing the polarization of the light, and the propagation direction of the light is changed by means of changing the propagation direction of light to a direction in which both the reflected and the transmitted lights can be utilized so that a light having a ratio of s and p components which is different from the incident light (s; x0-percent p; y0-percent) is available in the propagation direction of the reflected light.
However, it is found that the invention of the above co-pending application involves two problems. The first problem is that dark and bright stripes are generated by re-absorption of a portion of the exiting light by the light guide resulting in a reduced intensity of p component. As shown in FIG. 2, a structure of laminated acrylic plates is employed as a light guide for utilizing polarized component which was not utilized conventionally.
FIG. 3 shows a case where the acrylic plate makes an angle of 14-degrees with respect to a reflecting plate and a light is emitted from the upper surface at an angle of 70-degrees after total internal reflection at the bottom plate. While the light does not re-enter the light guide if the light emits out, as shown in FIG. 3, the light may re-enter the light guide right after exiting the light guide at the edge of neighboring acrylic plate depending on the positional relationship of emission. The re-entering light proceeds to the bottom surface and will never be emitted out.
Therefore, there are places where the light is totally emitted out and places where the light is re-absorbed depending on places, resulting in dark and bright stripes when observed as a liquid crystal display device. Naturally, the overall brightness is low due to the loss of the light.
The second problem is that the entire device is thick and heavy. In consideration of the refraction index of the acrylic sheet being 1.49 and that of air being 1.00, an angle coinciding the Brewster angle after total reflection at the bottom surface is limited to the case where the angle of the slope of the light guide is 28/m, where m is the number of total reflections of the light at the bottom surface until the light is emitted outwardly.
FIG. 3 shows the case of m=2 where the light is once reflected at the bottom surface and then reflected at the interface with the neighboring acrylic plate followed by a further total reflection at the bottom surface. The angle of slope will be 14-degrees in this case, and this would result in more than 30 mm thickness of the entire light guide plate when a model of 12-inches class is manufactured. This is a considerable disadvantage when taking into consideration the fact that the main stream of conventional applications in which the liquid crystal surface is illuminated by a non-polarized dispersed light is about 3 mm.
Further, the weight is heavy because the entire square cross section is filled with acrylic material as shown in FIG. 2. The light guide has to be made thinner and lighter for use in a liquid crystal display device. A conventional type of light guide using non-polarized dispersed light may be often formed into the shape of a wedge to dispose a circuit plate in the open space at the apex thereof for effective utilization of the space. Whereas, those shown in FIG. 2 and FIG. 3 which use polarized component not so far utilized have a problem in that they are of entire cross section of rectangular shape without having a tapered end in the side opposite to the side connected to the fluorescent lamp so that they are not compatible with a conventionally used wedge type light guide.
In addition to the above-described problems, the prior art technology has a problem as described below. Because the light is emitted out in an inclined direction from the light guide, it is necessary to change the direction of the inclined emitted light vertically with respect to the screen so that the brightness as seen from the front of the liquid crystal imaging device is maximized. Reduction of the weight of a liquid crystal imaging device requires that a light guide sheet makes a small angle with respect to a reflecting plate unit and this would result in a larger inclination of the angle of the direction of the emitted light. Conventionally, a prism was disposed with its apex directed to the direction opposite to the light guide, but a larger angle of inclination requires a sharper apex which is more difficult to manufacture.